Genetic control of male fertility in Arabidopsis thaliana: structural analyses of postmeiotic developmental mutants

The Regulatory Roles of MYC TFs in Plant Stamen Development

The stamen, as the male reproductive organ of flowering plants, plays a critical role in completing the life cycle of plants. MYC transcription factors are members of the bHLH IIIE subgroup and participate in a number of plant biological processes. In recent decades, a number of studies have confirmed that MYC transcription factors actively participate in the regulation of stamen development and have a critical impact on plant fertility. In this review, we summarized how MYC transcription factors play a role in regulating secondary thickening of the anther endothecium, the development and degradation of the tapetum, stomatal differentiation, and the dehydration of the anther epidermis. With regard to anther physiological metabolism, MYC transcription factors control dehydrin synthesis, ion and water transport, and carbohydrate metabolism to influence pollen viability. Additionally, MYCs participate in the JA signal transduction pathway, where they directly or indirectly control the development of stamens through the ET-JA, GA-JA, and ABA-JA pathways. By identifying the functions of MYCs during plant stamen development, it will help us to obtain a more comprehensive understanding not only on the molecular functions of this TF family but also the mechanisms underlying stamen development.

Editing melon eIF4E associates with virus resistance and male sterility

The cap-binding protein eIF4E, through its interaction with eIF4G, constitutes the core of the eIF4F complex, which plays a key role in the circularization of mRNAs and their subsequent cap-dependent translation. In addition to its fundamental role in mRNA translation initiation, other functions have been described or suggested for eIF4E, including acting as a proviral factor and participating in sexual development. We used CRISPR/Cas9 genome editing to generate melon eif4e knockout mutant lines. Editing worked efficiently in melon, as we obtained transformed plants with a single-nucleotide deletion in homozygosis in the first eIF4E exon already in a T0 generation. Edited and non-transgenic plants of a segregating F2 generation were inoculated with Moroccan watermelon mosaic virus (MWMV); homozygous mutant plants showed virus resistance, while heterozygous and non-mutant plants were infected, in agreement with our previous results with plants silenced in eIF4E. Interestingly, all homozygous edited plants of the T0 and F2 generations showed a male sterility phenotype, while crossing with wild-type plants restored fertility, displaying a perfect correlation between the segregation of the male sterility phenotype and the segregation of the eif4e mutation. Morphological comparative analysis of melon male flowers along consecutive developmental stages showed postmeiotic abnormal development for both microsporocytes and tapetum, with clear differences in the timing of tapetum degradation in the mutant versus wild-type. An RNA-Seq analysis identified critical genes in pollen development that were down-regulated in flowers of eif4e/eif4e plants, and suggested that eIF4E-specific mRNA translation initiation is a limiting factor for male gametes formation in melon.

Genome-Wide Analysis of Type-III Polyketide Synthases in Wheat and Possible Roles in Wheat Sheath-Blight Resistance

The enzymes in the chalcone synthase family, also known as type-III polyketide synthases (PKSs), play important roles in the biosynthesis of various plant secondary metabolites and plant adaptation to environmental stresses. There have been few detailed reports regarding the gene and tissue expression profiles of the PKS (TaPKS) family members in wheat (Triticum aestivum L.). In this study, 81 candidate TaPKS genes were identified in the wheat genome, which were designated as TaPKS1–81. Phylogenetic analysis divided the TaPKS genes into two groups. TaPKS gene family expansion mainly occurred via tandem duplication and fragment duplication. In addition, we analyzed the physical and chemical properties, gene structures, and cis-acting elements of TaPKS gene family members. RNA-seq analysis showed that the expression of TaPKS genes was tissue-specific, and their expression levels differed before and after infection with Rhizoctonia cerealis. The expression levels of four TaPKS genes were also analyzed via qRT-PCR after treatment with methyl jasmonate, salicylic acid, abscisic acid, and ethylene. In the present study, we systematically identified and analyzed TaPKS gene family members in wheat, and our findings may facilitate the cloning of candidate genes associated with resistance to sheath blight in wheat.

HMS1 interacts with HMS1I to regulate very-long-chain fatty acid biosynthesis and the humidity-sensitive genic male sterility in rice (Oryza sativa)

Environment-sensitive genic male sterility (EGMS) lines are used widely in two-line hybrid breeding in rice (Oryza sativa). At present, photoperiod-sensitive genic male sterility (PGMS) lines and thermo-sensitive genic male sterility (TGMS) lines are predominantly used in two-line hybrid rice, with humidity-sensitive genic male sterility (HGMS) lines rarely being reported. Here, it is shown that HUMIDITY-SENSITIVE GENIC MALE STERILITY 1 (HMS1), encoding a β-ketoacyl-CoA synthase, plays key roles in the biosynthesis of very-long-chain fatty acids (VLCFAs) and HGMS in rice. The hms1 mutant displayed decreased seed setting under low humidity, but normal seed setting under high humidity. HMS1 catalyzed the biosynthesis of the C26 and C28 VLCFAs, contributing to the formation of bacula and tryphine in the pollen wall, which protect the pollen from dehydration. Under low-humidity conditions, hms1 pollen showed poor adhesion and reduced germination on the stigmas, which could be rescued by increasing humidity. HMS1-INTERACTING PROTEIN (HMS1I) interacted with HMS1 to coregulate HGMS. Furthermore, both japonica and indica rice varieties with defective HMS1 exhibited HGMS, suggesting that hms1 potentially could be used in hybrid breeding. The results herein reveal the novel mechanism of VLCFA-mediated pollen wall formation, which protects pollen from low-humidity stress in rice, and has a potential use in hybrid crop breeding.

Abnormal tapetum development and energy metabolism associated with sterility in SaNa-1A CMS of Brassica napus L

Abnormal tapetum degradation and anther development in cytoplasmic male sterility SaNa-1A are the main reasons for the anther abortion. SaNa-1A is a novel cytoplasmic male sterility (CMS) line of Brassica napus derived from somatic hybrids of B. napus-Sinapis alba, and SaNa-1B is the corresponding maintainer line. Ultrastructural comparison between developing anthers of sterile and maintainer lines revealed abnormal subcellular structure of pollen mother cells (PMCs) in the CMS line. The PMC volume and size of nucleus and nucleolus in the CMS line were smaller than those in the maintainer line. The abnormal tapetum cell development and delayed tapetum degradation inhibited microspore development. Finally, anther abortion in the CMS line occurred. Physiological and biochemical analyses of developing anthers and mitochondria revealed that over-accumulation of reactive oxygen species (ROS) in the SaNa-1A and deficiency in antioxidant enzyme system aggravated the oxidization of membrane lipids, resulting in malondialdehyde (MDA) accumulation in anthers. High MDA content in the CMS line was toxic to the cells. ROS accumulation in SaNa-1A also affected anther development. Abnormal structure and function of terminal oxidase, which participates in the electron transport chain of mitochondrial membrane, were observed and affected the activity of cytochrome c oxidase and F₁F₀-ATPase, which inhibited ATP biosynthesis. Proline deficiency in SaNa-1A also affected anther development. Few hybridization signals of programmed cell death (PCD) in tetrads of SaNa-1A were identified using TdT-mediated dUTP Nick-End Labeling assay. PCD was not obvious in tapetum cells of SaNa-1A until the unicellular stage. These results validated the cytological differences mentioned above, and proved that abnormal tapetum degradation and anther development in SaNa-1A were the main reasons for the anther abortion.

Aberrant Meiotic Prophase I Leads to Genic Male Sterility in the Novel TE5A Mutant of Brassica napus

Genic male sterility (GMS) has already been extensively utilized for hybrid rapeseed production. TE5A is a novel thermo-sensitive dominant GMS line in Brassica napus, however, its mechanisms of GMS remain largely unclear. Histological and Transmission electron microscopy (TEM) analyses of anthers showed that the male gamete development of TE5A was arrested at meiosis prophase I. EdU uptake of S-phase meiocytes revealed that the TE5A mutant could accomplish DNA replication, however, chromosomal and fluorescence in situ hybridization (FISH) analyses of TE5A showed that homologous chromosomes could not pair, synapse, condense and form bivalents. We then analyzed the transcriptome differences between young floral buds of sterile plants and its near-isogenic fertile plants through RNA-Seq. A total of 3,841 differentially expressed genes (DEGs) were obtained, some of which were associated with homologous chromosome behavior and cell cycle control during meiosis. Dynamic expression changes of selected candidate DEGs were then analyzed at different anther developmental stages. The present study not only demonstrated that the TE5A mutant had defects in meiotic prophase I via detailed cytological analysis, but also provided a global insight into GMS-associated DEGs and elucidated the mechanisms of GMS in TE5A through RNA-Seq.

Transcriptome analysis of cytoplasmic male sterility and restoration in CMS-D8 cotton

A global view of differential expression of genes in CMS-D8 of cotton was presented in this study which will facilitate the understanding of cytoplasmic male sterility in cotton. Cytoplasmic male sterility (CMS) is a maternally inherited trait in higher plants which is incapable of producing functional pollen. However, the male fertility can be restored by one or more nuclear-encoded restorer genes. A genome-wide transcriptome analysis of CMS and restoration in cotton is currently lacking. In this study, Affymetrix GeneChips© Cotton Genome Array containing 24,132 transcripts was used to compare differentially expressed (DE) genes of flower buds at the meiosis stage between CMS and its restorer cotton plants conditioned by the D8 cytoplasm. A total of 458 (1.9 %) of DE genes including 127 up-regulated and 331 down-regulated ones were identified in the CMS-D8 line. Quantitative RT-PCR was used to validate 10 DE genes selected from seven functional categories. The most frequent DE gene group was found to encode putative proteins involved in cell wall expansion, such as pectinesterase, pectate lyase, pectin methylesterase, glyoxal oxidase, polygalacturonase, indole-3-acetic acid-amino synthetase, and xyloglucan endo-transglycosylase. Genes in cytoskeleton category including actin, which plays a key role in cell wall expansion, cell elongation and cell division, were also highly differentially expressed between the fertile and CMS plants. This work represents the first study in utilizing microarray to identify CMS-related genes by comparing overall DE genes between fertile and CMS plants in cotton. The results provide evidence that many CMS-associated genes are mainly involved in cell wall expansion. Further analysis will be required to elucidate the molecular mechanisms of male sterility which will facilitate the development of new hybrid cultivars in cotton.

Perspectives on deciphering mechanisms underlying plant heat stress response and thermotolerance

Global warming is a major threat for agriculture and food safety and in many cases the negative effects are already apparent. The current challenge of basic and applied plant science is to decipher the molecular mechanisms of heat stress response (HSR) and thermotolerance in detail and use this information to identify genotypes that will withstand unfavorable environmental conditions. Nowadays X-omics approaches complement the findings of previous targeted studies and highlight the complexity of HSR mechanisms giving information for so far unrecognized genes, proteins and metabolites as potential key players of thermotolerance. Even more, roles of epigenetic mechanisms and the involvement of small RNAs in thermotolerance are currently emerging and thus open new directions of yet unexplored areas of plant HSR. In parallel it is emerging that although the whole plant is vulnerable to heat, specific organs are particularly sensitive to elevated temperatures. This has redirected research from the vegetative to generative tissues. The sexual reproduction phase is considered as the most sensitive to heat and specifically pollen exhibits the highest sensitivity and frequently an elevation of the temperature just a few degrees above the optimum during pollen development can have detrimental effects for crop production. Compared to our knowledge on HSR of vegetative tissues, the information on pollen is still scarce. Nowadays, several techniques for high-throughput X-omics approaches provide major tools to explore the principles of pollen HSR and thermotolerance mechanisms in specific genotypes. The collection of such information will provide an excellent support for improvement of breeding programs to facilitate the development of tolerant cultivars. The review aims at describing the current knowledge of thermotolerance mechanisms and the technical advances which will foster new insights into this process.

Gametophytic vs. sporophytic control of pollen aperture number: a generational conflict

In flowering plants, the haploid phase is reduced to the pollen grain and embryo sac. These reproductive tissues (gametophytes) are actually distinct individuals that have a different genome from the plant (sporophyte), and are more or less independent. The morphology of pollen grains, particularly the openings permitting pollen tube germination (apertures), is crucial for determining the outcome of pollen competition. Many species of flowering plants simultaneously produce pollen grains with different aperture numbers in a single individual (heteromorphism). In this paper, we show that the heteromorphic pollen aperture pattern depends on the genetic control of pollen morphogenesis. This points out a conflict of interest between genes expressed in the sporophyte and genes expressed in the gametophyte. More generally, such a conflict should exist whenever heteromorphism is an ESS resulting from a bet-hedging strategy. For pollen aperture, heteromorphism has been observed in about 40% of angiosperm species, suggesting that conflicting situations are the rule. In this context, the sporo-gametophytic conflict could be one of the factors that led to the reduction of the haploid phase in plants.

ms17: a meiotic mutation causing partial male sterility in a corn silage hybrid

Cytological analysis under light microscopy of the single hybrid P30R50 of silage corn revealed an abnormal pattern of microsporogenesis that affected the meiotic products. Meiosis progressed normally until diakinesis, but before migration to the metaphase plate, bivalents underwent total desynapsis and 20 univalent chromosomes were scattered in the cytoplasm. At this stage, meiocytes also exhibited a number of chromatin-like fragments scattered throughout the cell. Metaphase I was completely abnormal in the affected cells, and univalent chromosomes and fragments were distributed among several curved spindles. Anaphase I did not occur, and each chromosome or group of chromosomes originated a micronucleus. After this phase, an irregular cytokinesis occurred, and secondary meiocytes with several micronuclei were observed. Metaphase II and anaphase II also did not occur, and after the second cytokinesis, the genomes were fractionated into polyads, generating several unbalanced microspores, with various-sized nuclei. About 35% of the tetrads were abnormal in the hybrid. This spontaneous mutation had been previously reported in a USA maize line called ms17 and was found to cause male sterility.

Temperature stress differentially modulates transcription in meiotic anthers of heat-tolerant and heat-sensitive tomato plants

BACKGROUND

Fluctuations in temperature occur naturally during plant growth and reproduction. However, in the hot summers this variation may become stressful and damaging for the molecular mechanisms involved in proper cell growth, impairing thus plant development and particularly fruit-set in many crop plants. Tolerance to such a stress can be achieved by constitutive gene expression or by rapid changes in gene expression, which ultimately leads to protection against thermal damage. We have used cDNA-AFLP and microarray analyses to compare the early response of the tomato meiotic anther transcriptome to moderate heat stress conditions (32°C) in a heat-tolerant and a heat-sensitive tomato genotype. In the light of the expected global temperature increases, elucidating such protective mechanisms and identifying candidate tolerance genes can be used to improve breeding strategies for crop tolerance to heat stress.

RESULTS

The cDNA-AFLP analysis shows that 30 h of moderate heat stress (MHS) alter the expression of approximately 1% of the studied transcript-derived fragments in a heat-sensitive genotype. The major effect is gene down-regulation after the first 2 h of stress. The microarray analysis subsequently applied to elucidate early responses of a heat-tolerant and a heat-sensitive tomato genotype, also shows about 1% of the genes having significant changes in expression after the 2 h of stress. The tolerant genotype not only reacts with moderate transcriptomic changes but also exhibits constitutively higher expression levels of genes involved in protection and thermotolerance.

CONCLUSION

In contrast to the heat-sensitive genotype, the heat-tolerant genotype exhibits moderate transcriptional changes under moderate heat stress. Moreover, the heat-tolerant genotype also shows a different constitutive gene expression profile compared to the heat-sensitive genotype, indicating genetic differences in adaptation to increased temperatures. In the heat-tolerant genotype, the majority of changes in gene expression is represented by up-regulation, while in the heat-sensitive genotype there is a general trend to down-regulate gene expression upon MHS. The putative functions associated with the genes identified by cDNA-AFLP or microarray indicate the involvement of heat shock, metabolism, antioxidant and development pathways. Based on the observed differences in response to MHS and on literature sources, we identified a number of candidate transcripts involved in heat-tolerance.

PIRL1 and PIRL9, encoding members of a novel plant-specific family of leucine-rich repeat proteins, are essential for differentiation of microspores into pollen

Plant intracellular Ras-group-related leucine-rich repeat proteins (PIRLs) are a plant-specific class of leucine-rich repeat (LRR) proteins related to animal and fungal LRRs that take part in developmental signaling and gene regulation. As part of a systematic functional study of the Arabidopsis thaliana PIRL gene family, T-DNA knockout mutants defective in the closely related PIRL1 and PIRL9 genes were identified and characterized. Pirl1 and pirl9 single mutants displayed normal transmission and did not exhibit an obvious developmental phenotype. To investigate the possibility of functional redundancy, crosses to generate double mutants were carried out; however, pirl1;pirl9 plants were not recovered. Reciprocal crosses between wild type and pirl1/PIRL1;pirl9 plants, which produce 50% pirl1;pirl9 gametophytes, indicated male-specific transmission failure of the double-mutant allele combination. Scanning electron microscopy and viability staining showed that approximately half of the pollen produced by pirl1/PIRL1;pirl9 plants was inviable and severely malformed. Tetrad analyses with qrt1 indicated that pollen defects segregated with the double-mutant allele combination, thus demonstrating that PIRL1 and PIRL9 function after meiosis. Pollen development was characterized with bright field, fluorescence, and transmission electron microscopy. Pirl1;pirl9 mutants stopped growing as microspores, failed to initiate vacuolar fission, aborted, and underwent cytoplasmic degeneration. Development consistently arrested at the late microspore stage, just prior to pollen mitosis I. Thus, PIRL1 and PIRL9 have redundant roles essential at a key transition point early in pollen development. Together, these results define a functional context for these two members of this distinct class of plant LRR genes.

Proteomics study of COI1-regulated proteins in Arabidopsis flower

Jasmonates (JAs) are a new class of plant hormone that regulate expression of diverse genes to mediate various plant responses. The Arabidopsis F-box protein COI1 is required for plant defense and male fertility in JA signal pathway. To further investigate the regulatory role of COI1 in male fertility, we compared the proteomics profiles of Arabidopsis wild type (WT) flowers with coi1-1 mutant male-sterile flowers using two-dimensional difference gel electrophoresis coupled with matrix-assisted laser desoption/ionization-time-of-flight mass spectrometry. Sixteen proteins with potential function in specific biological processes such as metabolism processes and defense/stress responses were differentially expressed in WT and coi1-1 mutant flowers. Verification on a phi class glutathione transferase AtGSTF9, one out of these 16 identified proteins, revealed that the expression of AtGSTF9 was severely downregulated in flowers of coi1-1 mutant compared with that of WT. Further function analyses of these genes would provide new insights into the molecular basis of COI1-regulated male fertility.

Gibberellin and jasmonate crosstalk during stamen development

Gibberellin (GA) and jasmonate (JA) are two types of phytohormones that play important roles during stamen development. For example, Arabidopsis plants deficient in either of GA or JA develop short stamens. An apparent question to ask is whether GA action and JA action during stamen filament development are independent of each other or are in a hierarchy. Recent studies showed that GA modulates the expression of genes essential for JA biosynthesis to promote JA production and high levels of JA will induce the expression of three MYB genes MYB21, MYB24 and MYB57. These three MYB genes are crucial factors for the normal development of stamen filament in Arabidopsis.

Premature tapetum degeneration: a major cause of abortive pollen development in photoperiod sensitive genic male sterility in rice

Photoperiod-sensitive genic male-sterile (PSGMS) rice (Oryza sativa L.), a natural mutant found in the rice cultivar Nongken 58, is very useful for the development of hybrid rice cultivars. Despite its widespread use in breeding programs, the initial stage of the abortive development of PSGMS rice and the possible cytological mechanisms of pollen abortion have not been determined. In the present study, a systematic cytological comparison of the anther development of PSGMS rice with its normal fertile counterpart is conducted. The results show that pollen abortion in PSGMS rice first occurs before the pollen mother cell (PMC) stage, and continues during the entire process of pollen development until pollen degradation. The abortive process was closely associated with the abnormal behavior of the tapetum. Although tapetum degeneration in PSGMS rice initiates already at the PMC stage, it proceeds slowly and does not complete until the breakdown of the pollen. Such cytological observations were supported by the results of the TUNEL (TdT-mediated dUTP Nick End Labeling) assay, which detects DNA fragmentation resulting from programmed cell death (PCD), indicating that the premature tapetum degeneration is in the process of PCD.

Gibberellin acts through jasmonate to control the expression of MYB21, MYB24, and MYB57 to promote stamen filament growth in Arabidopsis

Precise coordination between stamen and pistil development is essential to make a fertile flower. Mutations impairing stamen filament elongation, pollen maturation, or anther dehiscence will cause male sterility. Deficiency in plant hormone gibberellin (GA) causes male sterility due to accumulation of DELLA proteins, and GA triggers DELLA degradation to promote stamen development. Deficiency in plant hormone jasmonate (JA) also causes male sterility. However, little is known about the relationship between GA and JA in controlling stamen development. Here, we show that MYB21, MYB24, and MYB57 are GA-dependent stamen-enriched genes. Loss-of-function of two DELLAs RGA and RGL2 restores the expression of these three MYB genes together with restoration of stamen filament growth in GA-deficient plants. Genetic analysis showed that the myb21-t1 myb24-t1 myb57-t1 triple mutant confers a short stamen phenotype leading to male sterility. Further genetic and molecular studies demonstrate that GA suppresses DELLAs to mobilize the expression of the key JA biosynthesis gene DAD1, and this is consistent with the observation that the JA content in the young flower buds of the GA-deficient quadruple mutant ga1-3 gai-t6 rga-t2 rgl1-1 is much lower than that in the WT. We conclude that GA promotes JA biosynthesis to control the expression of MYB21, MYB24, and MYB57. Therefore, we have established a hierarchical relationship between GA and JA in that modulation of JA pathway by GA is one of the prerequisites for GA to regulate the normal stamen development in Arabidopsis.

A novel fatty Acyl-CoA Synthetase is required for pollen development and sporopollenin biosynthesis in Arabidopsis

Acyl-CoA Synthetase (ACOS) genes are related to 4-coumarate:CoA ligase (4CL) but have distinct functions. The Arabidopsis thaliana ACOS5 protein is in clade A of Arabidopsis ACOS proteins, the clade most closely related to 4CL proteins. This clade contains putative nonperoxisomal ACOS enzymes conserved in several angiosperm lineages and in the moss Physcomitrella patens. Although its function is unknown, ACOS5 is preferentially expressed in the flowers of all angiosperms examined. Here, we show that an acos5 mutant produced no pollen in mature anthers and no seeds by self-fertilization and was severely compromised in pollen wall formation apparently lacking sporopollenin or exine. The phenotype was first evident at stage 8 of anther development and correlated with maximum ACOS5 mRNA accumulation in tapetal cells at stages 7 to 8. Green fluorescent protein-ACOS5 fusions showed that ACOS5 is located in the cytoplasm. Recombinant ACOS5 enzyme was active against oleic acid, allowing kinetic constants for ACOS5 substrates to be established. Substrate competition assays indicated broad in vitro preference of the enzyme for medium-chain fatty acids. We propose that ACOS5 encodes an enzyme that participates in a conserved and ancient biochemical pathway required for sporopollenin monomer biosynthesis that may also include the Arabidopsis CYP703A2 and MS2 enzymes.

Functional analysis of the cellulose synthase-like genes CSLD1, CSLD2, and CSLD4 in tip-growing Arabidopsis cells

A reverse genetic approach was used to investigate the functions of three members of the cellulose synthase superfamily in Arabidopsis (Arabidopsis thaliana), CELLULOSE SYNTHASE-LIKE D1 (CSLD1), CSLD2, and CSLD4. CSLD2 is required for normal root hair growth but has a different role from that previously described for CSLD3 (KOJAK). CSLD2 is required during a later stage of hair development than CSLD3, and CSLD2 mutants produce root hairs with a range of abnormalities, with many root hairs rupturing late in development. Remarkably, though, it was often the case that in CSLD2 mutants, tip growth would resume after rupturing of root hairs. In silico, semiquantitative reverse transcription-polymerase chain reaction, and promoter-reporter construct analyses indicated that the expression of both CSLD2 and CSLD3 is elevated at reduced temperatures, and the phenotypes of mutants homozygous for insertions in these genes were partially rescued by reduced temperature growth. However, this was not the case for a double mutant homozygous for insertions in both CSLD2 and CSLD3, suggesting that there may be partial redundancy in the functions of these genes. Mutants in CSLD1 and CSLD4 had a defect in male transmission, and plants heterozygous for insertions in CSLD1 or CSLD4 were defective in their ability to produce pollen tubes, although the number and morphology of pollen grains was normal. We propose that the CSLD family of putative glycosyltransferases synthesize a polysaccharide that has a specialized structural role in the cell walls of tip-growing cells.

Temperature as a determinant factor for increased and reproducible in vitro pollen germination in Arabidopsis thaliana

Despite much effort, a robust protocol for in vitro germination of Arabidopsis thaliana pollen has been elusive. Here we show that controlled temperatures, a largely disregarded factor in previous studies, and a simple optimized medium, solidified or liquid, yielded pollen germination rates above 80% and pollen tube lengths of hundreds of microns, with both Columbia and Landsberg erecta (Ler) ecotypes. We found that pollen germination and tube growth were dependent on pollen density in both liquid and solid medium. Pollen germination rates were not substantially affected by flower or plant age. The quartet1 mutation negatively affected pollen germination, especially in the Ler ecotype. This protocol will facilitate functional analyses of insertional mutants affecting male gametophyte function, and should allow detailed gene expression analyses during pollen tube growth. Arabidopsis thaliana can now be included on the list of plant species that are suitable models for physiological studies of pollen tube elongation and tip growth.

Immunodetection of pectin and arabinogalactan protein epitopes during pollen exine formation of Beta vulgaris L

We present the results of ultrastructural and immunocytochemical studies of sugar beet microsporocytes during the developmental phase that begins with the first meiotic metaphase and ends with the formation of young tetrads. The most prominent feature noted during this period of microsporogenesis was the presence of numerous cisternae of endoplasmic reticulum which frequently lie perpendicular to the surface of the plasma membrane and eventually fuse to it. Microscopic observations have been combined with the detection of several carbohydrate epitopes representing pectins and arabinogalactan proteins in the primexine and incipient exine. Pectin domains that possess both low and highly methylesterified epitopes, as well as pectin side chains enriched in (1-->4)-beta-D-galactose residues, are deposited in this young microspore wall. The epitopes of arabinogalactan protein that bind to JIM13, JIM8, and LM2 antibodies are localised within the callose wall surrounding posttelophase tetrads. The possibility of endoplasmic-reticulum involvement in the synthesis, transport, or metabolism of several microspore wall compounds is discussed.

UDP-sugar pyrophosphorylase is essential for pollen development in Arabidopsis

Arabidopsis UDP-sugar pyrophosphorylase (AtUSP) is a broad substrate enzyme that synthesizes nucleotide sugars. The products of the AtUSP reaction can act as precursors for the synthesis of glycolipids, glycoproteins, and cell wall components including pectin and hemicellulose. AtUSP has no close homologs in Arabidopsis and its biological function has not been clearly defined. We identified two T-DNA insertional mutant lines for AtUSP, usp-1 and usp-2. No homozygous individuals were identified and progeny from plants heterozygous for usp-1 or usp-2 showed a 1:1 segregation ratio under selection. Despite decreased levels of both AtUSP transcript and USP activity (UDP-GlcA-->GlcA-1-P), heterozygous plants were indistinguishable from wild type at all stages of development. Reciprocal test crosses indicated the source of the segregation distortion was lack of transmission through the male gametophyte. Analysis of pollen tetrads from usp-1 in the quartet background revealed a 2:2 ratio of normal:collapsed pollen grains. The collapsed pollen grains were not viable as determined by Alexander's viability and DAPI staining, and pollen germination tests. The pollen phenotype of usp-1 was complemented by transformation of usp-1 with the AtUSP cDNA sequence. Surface and ultrastructural analyses of pollen from wild-type and usp mutants demonstrated that the mutation had no apparent effect on the outer wall (exine) but prevented the synthesis of the pectocellulosic inner wall (intine). Evidence presented here shows that AtUSP has a critical role in pollen development.

Regulation of Arabidopsis tapetum development and function by DYSFUNCTIONAL TAPETUM1 (DYT1) encoding a putative bHLH transcription factor

In flowering plants, male fertility depends on proper cell differentiation in the anther. However, relatively little is known about the genes that regulate anther cell differentiation and function. Here, we report the analysis of a new Arabidopsis male sterile mutant, dysfunctional tapetum1 (dyt1). The dyt1 mutant exhibits abnormal anther morphology beginning at anther stage 4, with tapetal cells that have excess and/or enlarged vacuoles and lack the densely stained cytoplasm typical of normal tapetal cells. The mutant meiocytes are able to complete meiosis I, but they do not have a thick callose wall; they often fail to complete meiotic cytokinesis and eventually collapse. DYT1 encodes a putative bHLH transcription factor and is strongly expressed in the tapetum from late anther stage 5 to early stage 6, and at a lower level in meiocytes. In addition, the level of DYT1 mRNA is reduced in the sporocyteless/nozzle (spl/nzz) and excess microsporocytes1/extra sporogenous cell (ems1/exs) mutants; together with the mutant phenotypes, this suggests that DYT1 acts downstream of SPL/NZZ and EMS1/EXS. RT-PCR results showed that the expression levels of many tapetum-preferential genes are reduced significantly in the dyt1 mutant, indicating that DYT1 is important for the expression of tapetum genes. Our results support the hypothesis that DYT1 is a crucial component of a genetic network that controls anther development and function.

Moderate increase of mean daily temperature adversely affects fruit set of Lycopersicon esculentum by disrupting specific physiological processes in male reproductive development

BACKGROUND AND AIMS

Global warming is gaining significance as a threat to natural and managed ecosystems since temperature is one of the major environmental factors affecting plant productivity. Hence, the effects of moderate temperature increase on the growth and development of the tomato plant (Lycopersicon esculentum) were investigated.

METHODS

Plants were grown at 32/26 degrees C as a moderately elevated temperature stress (METS) treatment or at 28/22 degrees C (day/night temperatures) as a control with natural light conditions. Vegetative growth and reproductive development as well as sugar content and metabolism, proline content and translocation in the androecium were investigated.

KEY RESULTS

METS did not cause a significant change in biomass, the number of flowers, or the number of pollen grains produced, but there was a significant decrease in the number of fruit set, pollen viability and the number of pollen grains released. Glucose and fructose contents in the androecium (i.e. all stamens from one flower) were generally higher in the control than METS, but sucrose was higher in METS. Coincidently, the mRNA transcript abundance of acid invertase in the androecium was decreased by METS. Proline contents in the androecium were almost the same in the control and METS, while the mRNA transcript level of proline transporter 1, which expresses specifically at the surface of microspores, was significantly decreased by METS.

CONCLUSIONS

The research indicated that failure of tomato fruit set under a moderately increased temperature above optimal is due to the disruption of sugar metabolism and proline translocation during the narrow window of male reproductive development.

Symmetric pollen mitosis I and suppression of pollen mitosis II prevent pollen development in Brachiaria jubata (Gramineae)

Microsporogenesis and pollen development were analyzed in a tetraploid (2n = 4x = 36) accession of the forage grass Brachiaria jubata (BRA 007820) from the Embrapa Beef Cattle Brachiaria collection that showed partial male sterility. Microsporocytes and pollen grains were prepared by squashing and staining with 0.5% propionic carmine. The meiotic process was typical of polyploids, with precocious chromosome migration to the poles and laggards in both meiosis I and II, resulting in tetrads with micronuclei in some microspores. After callose dissolution, microspores were released into the anther locule and appeared to be normal. Although each microspore initiated its differentiation into a pollen grain, in 11.1% of them nucleus polarization was not observed, i.e., pollen mitosis I was symmetric and the typical hemispherical cell plate was not detected. After a central cytokinesis, two equal-sized cells showing equal chromatin condensation and the same nuclear shape and size were formed. Generative cells and vegetative cells could not be distinguished. These cells did not undergo the second pollen mitosis and after completion of pollen wall synthesis each gave rise to a sterile and uninucleate pollen grain. The frequency of abnormal pollen mitosis varied among flowers and also among inflorescences. All plants were equally affected. The absence of fertile sperm cells in a considerable amount of pollen grains in this accession of B. jubata may compromise its use in breeding and could explain, at least in part, why seed production is low when compared with the amount of flowers per raceme.

Plants with increased expression of ent-kaurene oxidase are resistant to chemical inhibitors of this gibberellin biosynthesis enzyme

The gibberellin (GA) biosynthetic pathway includes the three-step oxidation of ent-kaurene to ent-kaurenoic acid, catalyzed by the enzyme ent-kaurene oxidase (KO). Arabidopsis plants overexpressing the KO cDNA under the control of the cauliflower mosaic virus 35S promoter, with or without a translational fusion to a modified green fluorescent protein (GFP), are very similar to wild-type (WT) plants under normal growth conditions. In contrast, when WT and 35S:KO (or 35S:KO-GFP) seeds, seedlings or pollen tubes are grown in the presence of chemical inhibitors of KO, such as paclobutrazol and uniconazole, plants with increased KO expression are partially resistant to the effects of these inhibitors. In combination with the observation that decreased KO levels increase the sensitivity to KO inhibitors, the 35S:KO phenotypes demonstrate that the modification of KO enzyme levels could be used to create transgenic crop plants with altered KO inhibitor response. These results also suggest that the KO gene could be used as a selectable marker for plant regeneration based on resistance to KO inhibitors. Finally, the observation that pollen tubes expressing 35S:KO or 35S:KO-GFP have decreased sensitivity to KO inhibitors provides further evidence for a physiological role for GAs in pollen tube elongation.

Molecular genetic analyses of microsporogenesis and microgametogenesis in flowering plants

In flowering plants, male reproductive development requires the formation of the stamen, including the differentiation of anther tissues. Within the anther, male meiosis produces microspores, which further develop into pollen grains, relying on both sporophytic and gametophytic gene functions. The mature pollen is released when the anther dehisces, allowing pollination to occur. Molecular studies have identified a large number of genes that are expressed during stamen and pollen development. Genetic analyses have demonstrated the function of some of these genes in specifying stamen identity, regulating anther cell division and differentiation, controlling male meiosis, supporting pollen development, and promoting anther dehiscence. These genes encode a variety of proteins, including transcriptional regulators, signal transduction proteins, regulators of protein degradation, and enzymes for the biosynthesis of hormones. Although much has been learned in recent decades, much more awaits to be discovered and understood; the future of the study of plant male reproduction remains bright and exciting with the ever-growing tool kits and rapidly expanding information and resources for gene function studies.

Pollen Development and Tube Growth are Affected in the Symbiotic Mutant of Lotus japonicus, crinkle

The symbiotic mutant of Lotus japonicus, crinkle (crk), exhibits abnormal nodulation and other alterations in the root hairs, trichomes, and seedpods. Defective nodulation in crk mutant is due to the arrested infection thread growth from the epidermis into the cortex. Here, we describe that crk is also affected in male fertility that causes the production of small pods with few seeds. Under in vitro conditions, pollen germination and tube growth were markedly reduced in the crk mutant. A swollen tip phenotype with disorganized filamentous actin (F-actin) was observed in the mutant pollen tubes after prolonged in vitro culture. During pollen development, the striking difference noted in the mutant was the small size of the microspores that remained spherical. Histological examination of ovule development, as well as outcrosses of the mutant as female to wild type as male, showed no evidence of abnormality in the female gametophyte development. Based on these findings, the Crk gene, aside from its role in the infection process during nodulation, is also involved in male gametophyte development and function. Therefore, this gene represents a connection between nodule symbiosis, polar tip growth, and other plant developmental processes.

Temporal and spatial distribution of pectin epitopes in differentiating anthers and microspores of fertile and sterile sugar beet

We studied the possible involvement of several pectin epitopes in anther differentiation and microsporogenesis in fertile and cytoplasmically male sterile sugar beets. The spatial and temporal distribution of five structural motifs were traced with a panel of monoclonal antibodies in six stages: premeiosis, meiotic prophase, young and mature tetrads, young and expanding microspores. The composition of the walls of sporogenous cells and meiocytes differed than that in the tapetum, as evidenced by the presence of alpha-Fuc(1-->2)-beta-Gal and alpha-(1-->5)-L-Ara epitopes binding CCRC-M1 and LM6 antibodies. At meiotic prophase, the meiocyte walls were additionally marked by the appearance of poorly methyl-esterified domains of homogalacturonan and of (1-->4)-beta-Gal residues, detected by JIM5 and LM5. Some constituents of the meiocyte wall which reacted with JIM5 and JIM7 persisted on the surface of the special callose sheath during tetrad development. In newly formed primexine and exine layers of tetrads and microspores, epitopes that were bound by JIM5, JIM7 and LM5 were abundant. No differences in the deposition or relative abundance of pectins were found between fertile and sterile anthers until microspore release from the callose. Later, at the time of abortion, sterile microspores had much larger amounts of epitopes detected by JIM5 than their fertile counterparts.

TheDUETgene is necessary for chromosome organization and progression during male meiosis inArabidopsisand encodes a PHD finger protein

Progression through the meiotic cell cycle is an essential part of the developmental program of sporogenesis in plants. The duet mutant of Arabidopsis was identified as a male sterile mutant that lacked pollen and underwent an aberrant male meiosis. Male meiocyte division resulted in the formation of two cells instead of a normal tetrad. In wild type, male meiosis extends across two successive bud positions in an inflorescence whereas in duet, meiotic stages covered three to five bud positions indicating defective progression. Normal microspores were absent in the mutant and the products of the aberrant meiosis were uni- to tri-nucleate cells that later degenerated, resulting in anthers containing largely empty locules. Defects in male meiotic chromosome organization were observed starting from diplotene and extending to subsequent stages of meiosis. There was an accumulation of meiotic structures at metaphase 1, suggesting an arrest in cell cycle progression. Double mutant analysis revealed interaction with dyad, a mutation causing chromosome cohesion during female meiosis. Cloning and molecular analysis of DUET indicated that it potentially encodes a PHD-finger protein and shows specific expression in male meiocytes. Taken together these data suggest that DUET is required for male meiotic chromosome organization and progression.

Disruption of Arabidopsis thaliana MYB26 results in male sterility due to non-dehiscent anthers

A male sterile mutant with a defect in anther dehiscence was identified in an Arabidopsis thaliana population mutagenized with the Zea mays transposon En-1/Spm. Mutants produce viable pollen that can fertilize when released mechanically from the anthers. Mutant stamens are of normal size and shape, but lack cell wall fortifications in the endothecial cell layer of the anther, which are required for the dehiscence process. The mutant phenotype was shown to be caused by a transposon insertion in AtMYB26, disrupting the putative DNA-binding domain of this R2R3-type MYB transcription factor. RT-PCR revealed that expression of AtMYB26 is restricted to inflorescences. Sterility was shown to be stable under several environmental conditions. The high stability of the sterile phenotype, together with the fact that pollen is functional, makes AtMYB26 and its orthologs a valuable tool for manipulating male fertility in higher plants.

Antisense inhibition of mitochondrial pyruvate dehydrogenase E1alpha subunit in anther tapetum causes male sterility

We hypothesized that cytoplasmic male sterility (CMS) in sugar beet may be the consequence of mitochondrial dysfunctions affecting normal anther development. To test the hypothesis, we attempted to mimic the sugar beet CMS phenotype by inhibiting the expression of mitochondrial pyruvate dehydrogenase (PDH), which is essential for the operation of the tricarboxylic acid (TCA) cycle. Screening with a cDNA library of sugar beet flower buds allowed the identification of two PDH E1alpha subunit genes (bvPDH_E1alpha-1 and bvPDH_E1alpha-2). bvPDH_E1alpha-1 was found to be highly expressed in tap roots, whereas bvPDH_E1alpha-2 was expressed most abundantly in flower buds. Green fluorescent protein (GFP) fusion of bvPDH_E1alpha revealed mitochondrial targeting properties. A 300-bp bvPDH_E1alpha-1 cDNA sequence (from +620 to +926) was connected to a tapetum-specific promoter in the antisense orientation and then introduced into tobacco. Antisense expression of bvPDH_E1alpha-1 resulted in conspicuously decreased endogenous bvPDH_E1alpha-1 transcripts and male sterility. The tapetum in the male-sterile anthers showed swelling or abnormal vacuolation. It is also worth noting that in the sterile anthers, cell organelles, such as elaioplasts, tapetosomes and orbicules were poorly formed and microspores exhibited aberrant exine development. These features are shared by sugar beet CMS. The results thus clearly indicate that inhibition of PDH activity in anther tapetum is sufficient to cause male sterility, a phenocopy of the sugar beet CMS.

Absence of microspore polarity, symmetric divisions and pollen cell fate in Brachiaria decumbens (Gramineae)

Meiotic division and male gametophyte development were analyzed in one tetraploid (2n = 4x = 36) accession of Brachiaria decumbens cv. Basilisk that showed some pollen sterility. Meiotic process was typical of polyploids in that it consisted of multiple chromosome associations. Precocious chromosome migration to the poles, laggards, and micronucleus formation were abundant in both meiosis I and II and resulted in tetrads with micronuclei. After callose dissolution, microspores were released into the anther locule and had the semblance of being normal. Although each microspore initiated its differentiation by pollen mitosis, in 43.24% of the microspores, nuclear polarization was not observed and the typical hemispherical cell plate was not detected. Division was symmetric and microspores lacked differentiation between the vegetative and the generative cell. Both nuclei were of equal size, presented equal chromatin condensation, and had a spherical shape. After the first pollen mitosis and cytokinesis, each cell underwent a new symmetric mitosis without nuclear polarization. At the end of the second pollen mitosis, four equal nuclei were observed in each pollen grain. After the second cytokinesis, the cells gave rise to four equal-sized pollen grains with a similar tetrad configuration that initially remained together. Sterile pollen grains resulted from abnormal pollen mitosis. This anomaly may be explained by a mutation, probably affecting microtubule cytoskeleton formation. The importance of this male-sterile mutation for Brachiaria breeding programs is discussed.

The Arabidopsis ABORTED MICROSPORES (AMS) gene encodes a MYC class transcription factor

Visual screening of a T-DNA mutagenised population of Arabidopsis thaliana for an absence of silique elongation lead to the isolation of the aborted microspores (ams) mutant that shows a sporophytic recessive male sterile phenotype. Homozygous mutant plants are completely devoid of mature pollen. Pollen degeneration occurs shortly after release of the microspores from the tetrad, prior to pollen mitosis I. Premature tapetum and microspore degeneration are the primary defects caused by this lesion, while a secondary effect is visualised in the stamen filaments, which are reduced in length and lie beneath the receptive stigma at flower opening. The disrupted gene was isolated and revealed a T-DNA element to be inserted into the eighth exon of a basic helix-loop-helix (bHLH) gene located on chromosome II. This protein sequence contains a basic DNA binding domain and two alpha helices separated by a loop, typical of a transcription factor belonging to the MYC sub family of bHLH genes. Therefore, AMS plays a crucial role in tapetal cell development and the post-meiotic transcriptional regulation of microspore development within the developing anther.

An original mutation in soybean (Glycine max (L.) Merrill) involving degeneration of the generative cell and causing male sterility

A spontaneous mutation causing male sterility has been detected in line BR97-17739 from the soybean breeding program conducted by Embrapa-National Soybean Research Center. Meiotic division and male gametophyte development were analyzed in 10 male-sterile, female-fertile plants. Meiotic process had few irregularities related to chromosome segregation and affected about 2% of tetrads. Despite the high frequency of normal microspores, pollen sterility was total. After callose dissolution, microspores were released into the anther loculle and interphase nucleus was displaced from the center to one side of the cell. Displacement continued throughout normal microspore mitosis (PMI). After telophase, the hemispherical phragmoplast marked the place of cytokinesis. A typical generative cell, adjacent to the plasma membrane, and the vegetative one, containing most of the cytoplasm, were formed. In spite of the well-formed generative cell, pollen mitosis (PMII) failed to occur. The generative cell degenerated and was completely destroyed. The 3:1 segregation for male sterility in this line and its progenies indicate that a single recessive gene controls mutation.

The excess microsporocytes1 gene encodes a putative leucine-rich repeat receptor protein kinase that controls somatic and reproductive cell fates in the Arabidopsis anther

Cell differentiation is essential for the development of multicellular organisms. In flowering plants, the haploid male gametophytes (pollen grains) are generated in the anther from reproductive cells called microsporocytes. Several types of somatic cells ensure successful pollen development, and thus reproduction. However, it is not clear what genes regulate the differentiation of these diverse, highly specialized cells in the anther. We report here the isolation and characterization of a novel Arabidopsis thaliana male sterile mutant, excess microsporocytes1 (ems1), that produces excess microsporocytes, lacks tapetal cells, and abnormally maintains middle layer cells. Although the meiotic nuclear division in the ems1 mutant is normal, the microsporocytes do not undergo cytokinesis, resulting in failed microsporogenesis and male sterility. The EMS1 gene encodes a putative leucine-rich repeat receptor protein kinase (LRR-RPK), and its expression is associated with the differentiation of the microsporocytes and tapetal cells, suggesting that EMS1 mediates signals that control the fate of reproductive cells and their contiguous somatic cells.

The Arabidopsis MALE STERILITY1 (MS1) gene is a transcriptional regulator of male gametogenesis, with homology to the PHD-finger family of transcription factors

We report here the molecular characterisation of the Arabidopsis MALE STERILITY1 gene, which is a critical sporophytic controlling factor for anther and pollen development. Homozygous ms1 mutants do not produce viable pollen, but are otherwise phenotypically normal. Degeneration of pollen occurs soon after microspore release from the tetrads, at which time the tapetum also appears abnormally vacuolated. The MS1 gene is expressed at low levels in anthers from closed buds, with expression in the tapetum at the stage of microspore release. No expression is seen in open flowers. The deduced MS1 protein sequence shows strong homology to the PHD-finger motif found in known transcription factors from humans, yeast and higher plants. Six alleles of ms1 have been identified; all result in premature termination of the MS1 protein and loss of the PHD-finger motif. MS1 is likely to play a key role in regulating transcription during specific stages of male gametogenesis and anther development. As such, MS1 provides a valuable tool for the manipulation of male sterility in higher plants.

Ultrastructural characterization of male sterile33 (ms33) mutant in Arabidopsis affected in pollen desiccation and maturation

The MS33 gene in Arabidopsis is required for stamen filament growth and for pollen maturation. The objective of this study was to characterize the effects of ms33 mutation on pollen development at the ultrastructural level. There were no differences between the wild type and ms33 mutant pollen development before the first mitotic division of microspores. At the bicellular pollen stage, the first signs of abnormalities were observed in the ms33 tapetum, which started to degenerate early and released osmiophilic material in the anther locule. In ms33 pollen, the endintine was thicker, and exintine thinner, than in the wild type, and the mutant pollen had large vacuoles. Later in development, the mutant pollen underwent second mitosis and produced two normal-looking sperm cells; however, the intine was precociously formed, and there were abnormalities in tryphine deposition on the pollen wall, in the size of vacuoles, and in the formation of lipid bodies in the vegetative cell cytoplasm. Based on these observations it is suggested that mutation in the MS33 gene interferes with intine formation and tryphine deposition, both of which negatively affect pollen desiccation resulting in large, highly vacuolate pollen that are nonviable.Key words: Arabidopsis, male sterility, mutant, pollen, tapetum, ultrastructure.

The Arabidopsis SKP1-LIKE1 gene is essential for male meiosis and may control homologue separation

The yeast and human SKP1 genes regulate the mitotic cell cycle but are not yet known to be required for meiosis. Nine Arabidopsis SKP1 homologues have been uncovered and are named ASK1 through ASK9. Here, we report the isolation and characterization of a male sterile Arabidopsis mutant and show that the mutant defect was caused by a Ds transposon insertion into the ASK1 gene. In the ask1-1 mutant, abnormal microspores exhibit a range of sizes. Furthermore, during mutant male meiosis, although homologous chromosome pairing appeared normal at metaphase I, chromosome segregation at anaphase I is unequal, and some chromosomes are abnormally extended. Therefore, in ask1-1, at least some homologues remain associated after metaphase I. In addition, immunofluorescence microscopy indicates that the mutant spindle morphology at both metaphase I and early anaphase I is normal; thus, the abnormal chromosome segregation is not likely caused by a spindle defect. Because the yeast Skp1p is required for targeting specific proteins for ubiquitin-mediated proteolysis, we propose that ASK1 controls homologue separation by degrading or otherwise removing a protein that is required directly or indirectly for homologue association before anaphase I.